Komatsuengine(no series)

Komatsu 6D125-1A(S/N 10001-UP)

Maintenance schedule, common problems & OEM parts breakdown

Search live 6D125-1A(S/N 10001-UP) parts

The 6D125-1A (S/N 10001-UP) is a 1980s-era mechanical, naturally aspirated inline-six diesel used as the base engine of the 6D125-1 family. It powers the Komatsu D50A-17 crawler dozer and its D50P-17 and D50PL-17 low-ground-pressure swamp variants, chassis S/N 60001-UP and 80001-UP, placing the machine in the small-to-medium crawler dozer class. Gross output for this pairing runs about 92-93 kW (124 hp) at 1750 rpm, and operating weight sits near 12,240 kg in base configuration, climbing higher on the wider-track LGP swamp models. The -17 series followed the D50A-16, which used a smaller four-cylinder engine, and was itself followed by the D50A-18, which carried the same 6D125-1 base engine at a higher rating. Configurations offered across the -17 range include the standard A-frame dozer, the P wide-track swamp variant, and the PL extra-wide LGP swamp variant, each available with straight or angle blade fitment.

The move from the D50A-16 to the D50A-17 marked a shift from a four-cylinder engine to the six-cylinder 6D125-1, a longer-stroke, smoother-running unit that Komatsu carried across a wide range of dozers, loaders, and other machines of the same era, giving the -17 series strong parts commonality with sibling models. The engine and its powertrain are entirely mechanical, with no onboard diagnostics, so gauges, caution lamps, and scheduled sampling do all the monitoring. That simplicity is exactly what keeps the 6D125-1A relevant in the used and parts market today: rental yards, small contractors, and secondary-market rebuilders favor these mechanical, pre-electronic engines because they can be serviced and repaired in the field without diagnostic tooling, using widely available common 6D125-family parts. Demand for the D50A-17 platform persists mainly in regions and applications where low-cost, easily-maintained mid-size dozers still do useful work long after newer, electronically-governed generations have taken over frontline fleets.

Below: full specifications, fluids & capacities, the factory service schedule, common service parts, verified fault codes, what owners discuss, attachment guidance, the complete assembly directory, and a serial-number reference. Complete parts lists with full OEM part numbers, exploded diagrams, quantities, and fitment data are available free in Heavy Parts AI.

Komatsu 6D125-1A(S/N 10001-UP) specifications

Engine

Model / series	6D125-1A, naturally aspirated inline-6 diesel in the Komatsu 6D125-1 family, S/N 10001-UP. Fitted across the D50, D53, D58, D60 and D65 dozer line from this serial start. Distinct from the turbocharged S6D125-1 and from the later 6D125-1AB/-1AF/-1AM/-1AN suffix engines used on other serial ranges.
Gross power	92-123 kW (124-165 hp) at 1750-1900 rpm, rating set by the host dozer model and configuration (standard vs. wide-track/LGP). Lower end on the D50A-17, upper end on the D60P-8.
Net power	Not published for this exact serial range in available literature.
Displacement	11.04 L (673.8 cu in / 11,042 cm3)
Cylinders, bore x stroke	6 cylinders in line, 125 mm x 150 mm bore/stroke (consistent with the published displacement figure for this engine)
Aspiration	Naturally aspirated, mechanical fuel injection
Emissions tier	Predates emissions-tier certification; no EPA/EU tier rating applies to this engine

Weights

Operating weight (installed, by host machine)	10,850-17,550 kg (23,920-38,690 lb), varies by dozer model and configuration (standard, LGP, swamp)
Shipping weight	Not documented separately from operating weight in available spec sheets
Ground pressure	Not documented for this exact serial range - verify with dealer or shop manual

Dimensions

Overall length (with blade, by host machine)	3.80-5.59 m (12.46-18.33 ft), varies by dozer model and blade
Width over tracks	2.34-3.00 m (7.68-9.85 ft), varies by configuration
Height to cab top	2.15-3.06 m (7.04-10.03 ft), varies by configuration
Track gauge	1.88-2.05 m (6.17-6.73 ft), varies by configuration
Ground clearance	0.34-0.51 m (1.10-1.68 ft), varies by configuration

Performance

Travel speed, forward	Approx. 9.8-10.9 km/h (6.1-6.8 mph), documented only for select host models; not published across the full application range
Travel speed, reverse	Approx. 11.9-13.8 km/h (7.4-8.6 mph), documented only for select host models; not published across the full application range
Rated engine speed	1750-1900 rpm depending on governed application
Gradeability	Not documented for this exact serial range - verify with dealer or shop manual

Service capacities (summary)

Fuel tank (by host machine)	250-320 L (66-85 gal), varies by dozer model; not a fixed engine-level figure
Engine oil (crankcase)	Not documented in available public spec sheets for this exact serial range - verify with shop manual or dealer
Cooling system	Not reliably documented for this exact serial range. Closely related earlier D-series generations of similar displacement ran roughly 42-63 L, but this figure is not confirmed as applicable to the 6D125-1A S/N 10001-UP - verify with dealer
Hydraulic system	Not an engine-level spec; capacity is set by the host machine's hydraulic circuit and not documented specifically against this engine

Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.

6D125-1A(S/N 10001-UP) fluids & capacities

System	Capacity	Recommended fluid
Machine identification	n/a	The 6D125-1A (S/N 10001-Up) engine is the powerplant fitted to the Komatsu D50A-17 and D50P-17/D50PL-17 crawler dozer (equipment S/N 60001-Up and 80001-Up), a 4-cycle, water-cooled, in-line 6-cylinder, direct-injection diesel of 11.04 L displacement (125 mm bore x 150 mm stroke), rated 92.5 kW (124 HP) at 1,750 rpm. This is a distinct engine build from the 6D125-1 fitted to the companion D53A-17/D53P-17 (early production, engine S/N 80001-82457), which later moves to the 6D125-2 engine at higher machine serials, and from the other 6D125-1AB/-1AF/-1AM/-1AN variants used on different grader and dozer platforms. Fluid data below is scoped to the D50A/P-17 chassis this engine serial range actually powers.
Engine crankcase (with filter)	Exact factory refill quantity is not published in the accessible factory literature for this serial range; the shop manual's dedicated fluid-capacity table was not available for direct confirmation. Verify against the machine's fuel/coolant/lubricant table or with a Komatsu dealer before servicing.	Komatsu Genuine Diesel Engine Oil, SAE 15W-40 for normal ambient temperatures; SAE 10W-30 is the Komatsu-approved grade for colder climates. Equivalent cross-reference: Chevron Delo 400 series diesel engine oil built to the same Komatsu engine-oil specification.
Cooling system	Exact refill quantity not confirmed for this serial range in available factory documentation; radiator is a "D-type" core per the factory specification sheet. Confirm capacity with the periodic maintenance chart before draining/refilling.	Komatsu Super Coolant, supplied as concentrate mixed 50/50 with water (extended-life, all-season, anti-corrosion coolant). No separate winter/summer concentration split is called out beyond the standard 50/50 mix; use a higher concentrate ratio only for extreme cold per Komatsu's coolant freeze-protection chart.
Fuel tank	Exact tank capacity not confirmed for this serial range in the accessible factory literature.	Automotive diesel fuel (ASTM D975 No. 2-D equivalent) for normal conditions; switch to a lighter No. 1-D-equivalent fuel in cold weather per Komatsu's ambient-temperature fuel guidance. Keep tank topped up at end of shift to reduce condensation.
Power train case - transmission, bevel gear shaft, steering clutch/brake (shared splash-lubricated sump)	Not separately published for this serial range. The D50A/P-17 uses a manually shifted spur-gear transmission (5 speeds forward, 4 reverse) with splash-type lubrication, sharing lubrication with the spiral bevel gear shaft and the wet multiple-disc steering clutch/band brake, per the factory specifications section; no standalone capacity figure was accessible.	Komatsu Powertrain & Micro Clutch Oil (drive-train fluid spec): SAE 10W in cold conditions, SAE 30 in normal/warm conditions. Equivalent cross-reference: Chevron Delo TorqForce in matching viscosity grade.
Final drives (each side)	Not separately published for this serial range. Final drive is a 2-stage spur-gear reduction with splash-type lubrication per the factory specifications section; Komatsu shop literature for this chassis does not show it as an independently drained/filled compartment separate from the power train case in the data reviewed.	Komatsu Powertrain & Micro Clutch Oil, same grade as the power train case (SAE 10W cold / SAE 30 normal), unless the periodic maintenance chart calls out a dedicated final-drive fill point with Komatsu Gear Lubricant (SAE 80W-90 or 85W-140, API GL-5 class) - confirm which applies before servicing.
Hydraulic system and tank (work equipment - blade lift/tilt circuit)	No standalone hydraulic tank capacity is published; the hydraulic tank is integrated into the control valve assembly, so there is no separate reservoir on this chassis. The gear-type hydraulic pump delivers 181 L/min at 1,750 rpm engine speed, with a maximum system pressure of 13.7 MPa (140 kgf/cm2).	Komatsu hydraulic oil (46AW hydraulic fluid spec), ISO VG 46. Equivalent cross-reference: Chevron Rando HD or Delo TorqForce hydraulic-oil equivalents in ISO 46 grade. A Komatsu biodegradable hydraulic fluid option also exists under the same specification for environmentally sensitive sites.
Grease (chassis, pins, bushings, blade linkage) - spec only	Not applicable (grease points serviced per periodic lubrication schedule, not a fill volume).	Komatsu Lithium-base Grease EP2 (G2-LI), NLGI 2, general-purpose chassis grease for normal temperatures. Equivalent cross-reference: Chevron/Delo Grease ESI EP, NLGI 2. Use a lower-consistency (arctic-grade) NLGI 1 or 0.5 lithium grease in sustained sub-freezing climates per standard Komatsu cold-weather guidance.

Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.

Komatsu 6D125-1A(S/N 10001-UP) maintenance schedule

Service interval	Tasks
Every 10 h	Check engine oil level and coolant level before starting and top off as needed Drain water and sediment from the fuel tank sediment cup and water separator Check the air cleaner dust/clog indicator and service the element if flagged Walk around the machine and check for fuel, oil, and coolant leaks at the engine, steering-clutch case, and final drives Check undercarriage track tension and grease blade-lift cylinder pins and track-frame pivot points Confirm oil pressure, coolant temperature, torque-converter oil temperature, and water-in-fuel caution lamps and buzzer function at startup
Every 250 h	Change engine oil and replace the engine oil filter cartridge Check fan belt tension and condition and adjust deflection Check battery electrolyte level and clean/tighten cable terminals Check powertrain case oil level in the splash-lubricated steering-clutch housing and top off Grease equalizer bar, track-frame pivots, and steering-clutch linkage points Clean the fuel tank strainer and check the fuel filter bowl for water contamination
Every 500 h	Replace the fuel filter cartridge Check and adjust track shoe tension and inspect sprocket and idler wear Check final drive case oil level on both sides of the splash-lubricated two-stage spur-gear final drives Check steering clutch and brake pedal free travel and adjust linkage Sample or test coolant additive concentration Check hydraulic oil level in the control-valve-integrated tank
Every 1,000 h	Replace powertrain (steering-clutch case) oil Replace final drive case oil on both final drives Replace the hydraulic return filter element serving the pump circuit Check and adjust valve clearance on the inline-six cylinder head Inspect alternator and starter brushes and wiring connections Service or replace the air cleaner element
Every 2,000 h	Replace hydraulic tank oil in the control-valve-integrated reservoir Check and adjust fuel injection pump timing and inspect injector nozzles Inspect intake and exhaust manifolds for leaks and cracks Retorque cylinder head bolts and check cylinder compression Inspect steering clutch and brake linings for wear and adjust or reline as needed Check torque converter charging pump output pressure
Every 4,000 h	Replace engine coolant and flush the cooling system Inspect undercarriage track links, bushings, and sprockets against wear limits and turn or replace as needed Send the fuel injection pump and injectors for bench calibration or overhaul Fully inspect steering clutch and brake assemblies and reline or rebuild if worn past limit

Servicing the 6D125-1A(S/N 10001-UP) beyond the schedule

Predictive maintenance & fluid analysis

With no fault codes on the 6D125-1A, prevention rides on scheduled oil and coolant sampling plus disciplined gauge watching. Track engine oil condition for early wear metal in the splash-lubricated steering-clutch case and final drives, since neither system has its own filtration to mask contamination. Watch the oil pressure, coolant temperature, torque-converter oil temperature, and charge-pressure gauges and caution lamps at every start, and log trends rather than waiting for a lamp to trip before acting.

Corrective & common repairs

Typical failure points on this powertrain include worn steering-clutch and brake linings, contamination-driven wear in the splash-lubricated final drive gear sets, and leaks at the hydraulic control-valve assembly that also serves as the tank. The mechanical fuel injection pump and injectors drift out of calibration with hours and heat rather than throwing a code, so rough running or smoke is often the first sign. Track roller and idler wear is common given the undercarriage duty this dozer class sees.

Overhaul & rebuild points

Overhaul work on the 6D125-1A centers on cylinder liner and piston wear, injection pump and nozzle recalibration, and steering-clutch relining once linings reach the wear limit. Final drive gear sets and cases warrant inspection at major rebuild given their splash-lubrication and lack of filtration. Undercarriage components (track links, bushings, sprockets) typically drive rebuild timing on the D50A-17 platform as much as the engine itself, and torque-converter internal wear should be checked whenever the powertrain is opened.

Seasonal & environment servicing

In cold conditions, check coolant concentration and battery capacity closely, since this naturally aspirated mechanical engine has no electronic cold-start assist beyond glow or ether aids. In hot, dusty environments, shorten air cleaner service intervals and keep the radiator core clear of debris. On the D50P-17/D50PL-17 swamp configurations, track tension and undercarriage checks differ from the standard D50A-17 given the wider, lower-ground-pressure track and LGP undercarriage geometry.

6D125-1A(S/N 10001-UP) common service parts

Part number	Part	Notes
07281-00197	Clamp,(With Pre-Fuel Filter)	Fuel system service	Check fitment →
ND092220-0040	Filter,Fuel	Fuel system service	Check fitment →
09637-02161	Plate? Instruction,Air Cleaner (Double Element)	Air intake service	Check fitment →
600-181-2500	Element Ass'Y,Outer	Scheduled service item	Check fitment →
6114-80-7101	Element Ass'Y	Scheduled service item	Check fitment →

Always confirm against your machine's serial number — cross-check any part in Heavy Parts AI before ordering.

6D125-1A(S/N 10001-UP) attachments & work tools

Host machine and class

The 6D125-1A(S/N 10001-Up) engine is the power unit for the D50A-17 crawler dozer and its D50P-17/D50PL-17 LGP swamp variant. This places the platform in Komatsu's small-to-medium crawler dozer class, rated around 92-93 kW (124 hp) gross, with operating weight roughly 12,240 kg (27,000 lb) for the standard A-suffix machine.

Dozer blades

Factory blade offerings for this class run straight, angle, and semi-U configurations. The standard blade measures approximately 3.7 m (12.3 ft) wide by 0.88 m (34.5 in) high. Power-angle-tilt (PAT) hydraulic blade control is the common factory option for added versatility on dozers this size; exact blade capacity in volume terms is not consistently published and should be confirmed on the dealer spec sheet.

Rippers

Single-shank and multi-shank (three-shank) ripper attachments are offered for this dozer family, using hydraulically controlled parallelogram linkage with adjustable digging angle. This matches Komatsu's general practice for small/mid-class dozer rippers of this era. Penetration depth and shank spacing are not documented for this exact serial range; verify with your dealer.

Coupler/mounting system

No quick-attach coupler system applies to this machine class. Blades and rippers mount through factory-integrated push-arm and linkage assemblies rather than a quick-change coupler, standard for dozers of this size and vintage.

Hydraulic kit / arrangement notes

Blade angle/tilt and ripper functions run on factory-integrated hydraulic circuits built into the base machine. No auxiliary hydraulic kit is documented as a field-add requirement for attachments on this model, unlike excavator or loader tool circuits which often need aftermarket auxiliary plumbing.

Factory configuration / guarding options

The main documented factory configuration split is standard (A-suffix) versus LGP/swamp (P-suffix, sometimes PL) undercarriage, the latter using wider track shoes and a heavier operating weight near 14,400 kg (31,700 lb) for improved flotation. No specific counterweight, cab guarding, or ride-control option is documented for this model in available factory literature.

All 6D125-1A(S/N 10001-UP) assemblies by section

Every catalogued assembly group for the Komatsu 6D125-1A(S/N 10001-UP). Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.

Cooling System

Komatsu : 6d125-1a(S/N 10001-Up) : Cooling System : Cooling Fan And Mounting

01***25	Bolt,(For Canvas Canopy?Head Guard)	6
01***30	Bolt,(For Quick Shift) (Mitsubishi)	10
01***40	Bolt, Mount Master Brake Cylinder	8
01***95	Bolt,(For 2nd Travel Speed) (Op)	4

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Komatsu : 6d125-1a(S/N 10001-Up) : Cooling System : Corrosion Resistor And Piping

01***20	Bolt,(For Quick Shift) (Mitsubishi)	1
01***30	Bolt,(For Quick Shift) (Mitsubishi)	4
01***08	Nut,(For Canvas Canopy?Head Guard)	1
01***16	Washer,Radio Controlled Slag Handling	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Cooling System : Water Pump And Mounting

01***30	Bolt,(For France Noise Suppression)	2
01***16	Bolt (With Air Heater & Conditioner)	6
01***30	Bolt,(For Quick Shift) (Mitsubishi)	5
01***35	Bolt,(For Quick Shift) (Mitsubishi)	1

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Cylinder Block

Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Camshaft And Tappet

01***22	Bolt	2
04***25	Key, Plain Woodruff	1
04***12	Ring,Snap (For Ripper)	2
61***31	Plate,Thrust	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Crankshaft

01***50	Bolt,(For Strengthened Cover)	6
01***15	Bolt	1
01***20	Bolt	5
04***28	Pin, Dowel - Transmission Case, Pin-Dowel	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Cylinder Block

01***12	Bolt,(Without Air Compressor Type)	2
01***45	Washer,(For Travel Motor) (Not Show)	14
04***14	Pin, Dowel	2
04***20	Pin,Dowel (Welded)	12

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Cylinder Block Cover

01***20	Bolt,(For Quick Shift) (Mitsubishi)	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	1
01***08	Nut,(For Canvas Canopy?Head Guard)	1
01***16	Washer,Radio Controlled Slag Handling	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Flywheel And Flywheel Housing

01***40	Bolt,(Dust And Dirt Protection)	2
01***45	Bolt,(For Slag Handling)	2
01***50	Bolt, Radiator To Frame	2
01***70	Bolt - Thrust Plate Mounting	4

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Front Cover(#10001-34750)

01***25	Bolt,(Without Auto Priming System)	6
01***60	Screw	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	1
01***65	Bolt,(Except Japan)	8

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Front Cover(#34751-)

01***25	Bolt,(Without Auto Priming System)	6
01***60	Screw	1
01***65	Bolt,(Except Japan)	8
01***75	Bolt,(For Adjusting)	6

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Oil Pan

01***25	Bolt,(For Canvas Canopy?Head Guard)	29
01***30	Bolt,(For Quick Shift) (Mitsubishi)	7
01***16	Washer,Radio Controlled Slag Handling	36
01***32	Washer,(With Auto Emergency Steering)	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Oil Pan (For Trimming Dozer)

01***25	Bolt,(For Canvas Canopy?Head Guard)	29
01***30	Bolt,(For Quick Shift) (Mitsubishi)	7
01***16	Washer,Radio Controlled Slag Handling	35
01***32	Washer,(With Auto Emergency Steering)	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Piston And Connecting Rod

02***13	Pin, Dowel	2
04***18	Ring, Snap	2
61***10	Piston Ass'Y	6
61***11	Piston Ass'Y	6

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Power Take Off (1/2)

01***20	Bolt,(With Auto Emergency Steering)	6
01***25	Bolt,(For Canvas Canopy?Head Guard)	4
01***14	Nut,Ring	2
01***23	Washer,Lock (K5)	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Block : Power Take Off (2/2)

01***20	Bolt,(With Auto Emergency Steering)	6
01***25	Bolt,(For Canvas Canopy?Head Guard)	8
01***30	Bolt,(For Quick Shift) (Mitsubishi)	4
01***45	Bolt, Mounting Bracket To Radiator (Top)	7

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Cylinder Head

Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Air Cleaner And Mounting

01***25	Bolt,(For Canvas Canopy?Head Guard)	2
01***30	Bolt,(For Quick Shift) (Mitsubishi)	2
01***40	Bolt, Mount Master Brake Cylinder	1
01***08	Nut,(For Straight Tilt Dozer) (D65p)	6

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Air Cleaner And Mounting (For Trimming Dozer)

01***25	Bolt,(For Canvas Canopy?Head Guard)	2
01***30	Bolt,(For Quick Shift) (Mitsubishi)	2
01***08	Nut,(For Straight Tilt Dozer) (D65p)	6
01***16	Washer,Radio Controlled Slag Handling	6

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Air Cleaner And Mounting (Oil Path Type)(#10628-)

01***35	Bolt,(For Quick Shift) (Mitsubishi)	6
01***70	Nut,Wing	1
01***08	Nut,(For Canvas Canopy?Head Guard)	6
01***16	Washer,Radio Controlled Slag Handling	10

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Air Intake Manifold And Connection

01***16	Bolt, Mount Washer Tank To Bracket, Bolt	2
01***30	Bolt,(For Quick Shift) (Mitsubishi)	15
01***55	Bolt, Mount Tube To Switch Pump	6
01***60	Bolt, Mount Tube To Switch Pump	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Cylinder Head(#10001-26915)

04***20	Pin,Dowel (Welded)	2
61***30	Sleeve,Holder	1
61***20	Cotter, Valve	8
61***40	Seal,Exhaust Valve (K1)	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Cylinder Head(#26916-)

04***20	Pin,Dowel (Welded)	2
61***30	Sleeve,Holder	1
61***20	Cotter, Valve	8
61***40	Seal,Exhaust Valve (K1)	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Exhaust Manifold

01***80	Bolt, Mount Strainer Flange	18
01***00	Bolt (M10-1.50x100)	18
07***08	Plug,(Without Auto Priming System)	2
61***20	Spacer,(For Shipping)	18

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Muffler And Mounting(#10001-28436)

01***25	Bolt,(Sandy? Dusty Terrain Spec.)	4
01***40	Bolt,(Dust And Dirt Protection)	4
01***35	Bolt,(For Quick Shift) (Mitsubishi)	1
01***30	Bolt, Mount Pump	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Muffler And Mounting(#28437-)

01***25	Bolt,(Sandy? Dusty Terrain Spec.)	4
01***40	Bolt,(Dust And Dirt Protection)	4
01***35	Bolt,(For Quick Shift) (Mitsubishi)	1
01***30	Bolt, Mount Pump	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Rocker Arm And Housing

01***08	Nut, Regular Hexagon (M10-1.50)	6
01***08	Nut,R.H. Thread	6
01***16	Washer,Radio Controlled Slag Handling	40
01***32	Washer,(With Auto Emergency Steering)	40

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Rocker Arm Housing Cover

01***25	Bolt,(For Canvas Canopy?Head Guard)	1
01***08	Nut,(For Canvas Canopy?Head Guard)	1
01***16	Washer,Radio Controlled Slag Handling	25
07***50	Cap,Oil Filler	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Cylinder Head : Water Manifold And Thermostat

02***43	O-Ring,Water Manifold	10
07***12	Washer, Gasket,(Without Thermo Sensor) (K1)	1
07***08	Plug,(Without Auto Priming System)	1
07***12	Plug,(Without Corrosion Resistor)	2

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Electrical System

Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Alternator (13a)

60***50	Alternator A. (13a)	1
KD***31	Bushing,Insulator	2
KD***80	Packing	1
KD***72	Slip Ring Ass'Y	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Alternator (13a) (Sandy Dusty Spec.)

60***70	Alternator A. (13a),(Sandy And Dusty Terrain S.)	1
KD***31	Bushing,Insulator	2
KD***72	Slip Ring Ass'Y	1
KD***90	Wire,Read	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Alternator (35a)(#10628-)

60***20	Alternator A. (35a),(With Hardening Pulley)	1
KD***57	Bearing,Ball? Front	1
KD***80	Collar	1
KD***70	Rear Bracket Ass'Y,Rear	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Alternator And Mounting

01***55	Bolt, Mount Tube To Switch Pump	1
01***25	Bolt	1
01***11	Nut (Welded)	1
01***26	Washer,(For Additional Rear Lamp)	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Alternator And Mounting (For Anti-Corrosive Alternator)

01***55	Bolt, Mount Tube To Switch Pump	1
01***25	Bolt	1
01***11	Nut (Welded)	1
01***26	Washer,(For Additional Rear Lamp)	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Heater Switch And Heater Signal

60***70	Switch,Air Intake Heater	1
60***30	Signal,Engine Pre Heater	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Starting Motor (7.5kw) (1/2)

60***70	Starting Motor A.,(7.5kw) (See Fig.A6210-B4a4)	1
KD***20	Armature Ass'Y	1
KD***51	Switch Ass'Y,Safety Relay	1
KD***50	Rear Bracket Ass'Y	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Starting Motor (7.5kw) (2/2)

KD***60	Plunger Ass'Y	1
KD***30	Holder	1
KD***20	Screw	2
KD***01	Washer,(Not Shown)	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Starting Motor (7.5kw)(#10001-18199)

60***30	Starting Motor Ass'Y,(7.5kw)(See Fig.0661)	1
KD***20	Magnetic Switch Assembly	1
KD***40	Spring,Spring	4
KD***50	Brush Assembly	4

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Starting Motor (7.5kw)(#18200-)

60***30	Starting Motor Ass'Y,(7.5kw)	1
KD***60	Shift Lever Assembly	1
KD***40	Spring,Spring	4
KD***00	Terminal Assembly	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Starting Motor And Mounting

01***60	Bolt,(Shim Thickness?More Than 8mm)	3
66***10	Gasket,Starting Motor	1
(6***0)	Starting Motor Ass'Y,(7.5kw) (See Fig.0661a)	1
(6***0)	Starting Motor Ass'Y,(7.5kw) (See Fig.0661)	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Electrical System : Starting Motor And Mounting (For Trimming Dozer)

01***40	Bolt,(With Center Guard)	3
60***20	Clip,(Large Capacity Type)	1
66***10	Gasket,Starting Motor	1
(6***0)	Starting Motor Ass'Y,(7.5kw) (See Fig.A6210-B4a4)	1

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Engine Kit

Komatsu : 6d125-1a(S/N 10001-Up) : Engine Kit : Gasket Kit

01***23	Washer,Lock (K5)	3
02***43	O-Ring,Water Manifold	10
07***10	O-Ring,(F4350-62a0 -5 ), O-Ring; Komatsu S/N 22***01 & Abv - Was 07000-02010, O-Ring; Was 07000-02010	1
07***15	O-Ring,(For Low Speed) (D40pf)	1

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Fuel System

Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Filter And Piping

01***25	Bolt,(For Canvas Canopy?Head Guard)	2
01***30	Bolt,(For Quick Shift) (Mitsubishi)	4
01***35	Bolt,(For Quick Shift) (Mitsubishi)	2
01***16	Washer,Radio Controlled Slag Handling	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Coupling)(#10001-37407)

ND***50	Bolt	1
ND***70	Bolt	2
ND***30	Coupling Ass'Y	1
ND***60	Bolt	6

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Coupling)(#37408-)

ND***10	Flange,Coupling	1
ND***50	Bolt	1
ND***40	Bolt	2
ND***61	Bolt	6

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Feed Pump)

ND***40	Washer? 1mm,Plate	1
ND***00	Pump Ass'Y,Priming Pump	1
ND***70	Washer,Union	4
ND***10	Spring, Check Valve	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2)(#10001-37407)

ND***40	Plug,Screw	1
ND***50	Washer? 0.3mm	0
ND***30	Governor Ass'Y	1
ND***90	Lever Ass'Y,Control	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2)(#37408-)

ND***40	Plate,Thrust	1
ND***40	Plug,Screw	1
ND***70	Adapter Ass'Y	1
ND***10	Adapter	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2)(#10001-37407)

ND***20	Washer? 1.0mm,Cover Plate	1
ND***60	O-Ring	2
ND***70	Packing,Rubber Plate	1
ND***10	Seal,Oil	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2)(#37408-)

ND***20	Washer? 1.0mm,Cover Plate	2
ND***90	Seal,Oil	1
ND***40	O-Ring	1
ND***50	Packing,Rubber Plate	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Pump) (1/2)

61***11	Injection Pump A	1
61***12	Injection Pump A	1
61***13	Injection Pump A	1
ND***50	Ring,Adjust	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Pump) (2/2)

ND***70	O-Ring	6
ND***71	Rack,Control	1
ND***10	Washer	2
ND***50	Seat,Spring? Lower	6

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (1/2)

01***75	Bolt,(See Fig.0142)	4
01***25	Bolt,(For Canvas Canopy?Head Guard)	4
01***45	Bolt,(For Power Tilt Mech Angle)	2
01***70	Bolt, Mount Bracket And Oil Inlet Tube, Bolt	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (2/2)(#10001-29955)

01***30	Bolt	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	2
01***55	Bolt, Mount Tube To Switch Pump	4
01***19	Washer,Spring (With Extinguisher) (D31p)	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (2/2)(#29956-)

01***30	Bolt	1
01***20	Bolt,(For Quick Shift) (Mitsubishi)	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	1
01***55	Bolt, Mount Tube To Switch Pump	4

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Fuel Nozzle And Spill Pipe

01***45	Bolt, Divider Valve Mounting	12
07***65	O-Ring,Cylinder Block	6
07***12	Gasket,Mechanical Pump Piping	2
07***12	Gasket,(Without Air Compressor Type) (K2)	12

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Komatsu : 6d125-1a(S/N 10001-Up) : Fuel System : Water Separator And Mounting(#10628-)

01***25	Bolt,Without Revolution Sensor	2
01***20	Bolt,(For Quick Shift) (Mitsubishi)	2
01***16	Washer,(For 35a Alternator)	2
07***60	O-Ring,(For Pc750?800?1800) (A2610)	1

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Lubricating Oil System

Komatsu : 6d125-1a(S/N 10001-Up) : Lubricating Oil System : Lubricating Oil Cooler

01***25	Bolt,Without Revolution Sensor	2
01***25	Bolt,(For Canvas Canopy?Head Guard)	13
01***65	Bolt,(Except Japan)	2
01***80	Bolt,(Not Shown)	2

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Komatsu : 6d125-1a(S/N 10001-Up) : Lubricating Oil System : Lubricating Oil Filter And Mounting

01***25	Bolt,(For Canvas Canopy?Head Guard)	2
01***30	Bolt,(For Quick Shift) (Mitsubishi)	4
01***35	Bolt,(For Quick Shift) (Mitsubishi)	2
01***16	Washer,Radio Controlled Slag Handling	4

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Komatsu : 6d125-1a(S/N 10001-Up) : Lubricating Oil System : Lubricating Oil Piping

01***16	Bolt, Battery To Frame Ground Wire Attaching	1
01***35	Bolt,(For Quick Shift) (Mitsubishi)	2
01***50	Bolt,(With Service Valve R.H.)	2
01***85	Bolt,(For 2nd Travel Speed) (Op)	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Lubricating Oil System : Lubricating Oil Pump And Suction Pipe

01***25	Bolt,(Without Auto Priming System)	4
01***20	Bolt,(Large Capacity Type) (Std)	2
01***25	Bolt,(Sandy? Dusty Terrain Spec.)	5
01***35	Bolt, Mounting - Flywheel Housing End	1

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Komatsu : 6d125-1a(S/N 10001-Up) : Lubricating Oil System : P.T.O. Lubricating Oil Piping

01***25	Bolt,(For Canvas Canopy?Head Guard)	2
07***12	Gasket,(Without Air Compressor Type) (K2)	22
61***60	Bolt,Joint	6
61***70	Bolt,Joint	5

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6D125-1A(S/N 10001-UP) serial number reference

The 6D125-1 series dataplate sits on the left side of the engine, on or near the valve cover; if the tag is missing or unreadable, the serial is also stamped directly into the block casting. No alpha prefix ahead of the running serial has been found for this engine family in available documentation - the letter/number suffix tacked onto "6D125-1" (A, C, AB, AF, AM, AN, DD, EE, etc.) identifies the build/fitment configuration, while the digits that follow are the sequential production serial.

Prefix	Identifies	Notes
10001-UP (no alpha PIN prefix found ahead of the serial)	6D125-1A base spec - naturally aspirated, fitted to Komatsu D50A-17 (and D50P-17/D50PL-17) crawler dozer	Engine 6D125-1A, S/N 10001-Up, is the powerplant listed against the D50A-17 crawler dozer (chassis S/N 60001-Up and 80001-Up) and its D50P-17/D50PL-17 swamp variant (chassis S/N 60001-Up) - one engine, one starting serial, two chassis families. Gross power runs about 92.5 kW (124 hp) at 1750 rpm, consistent with a naturally aspirated block (Komatsu's plain '6D125' designation, without a leading S, denotes non-turbo; the turbocharged sibling carries the S6D125 designation). The closely related D53A-17/D53P-17 dozer on the same chassis platform uses a different engine spec, 6D125-1C, not 6D125-1A. Exact plant/region of manufacture and production calendar years are not documented - confirm with a dealer. Sibling suffix builds on the same 6D125-1 base (1AB, 1AB-N, 1AF, 1AM/1AN, 1C on the D53A-17, and others) are separate parts-book entries tied to other dozer/grader/loader fitments and different serial start points - do not cross-apply without dealer confirmation.

Frequently asked questions

What engine powers the 6D125-1A (S/N 10001-UP)?

It is a naturally aspirated, mechanical (non-electronic) 11.04 L inline-six diesel with 125 x 150 mm bore and stroke, rated about 92-93 kW (124 hp) at 1750 rpm in its Komatsu D50A-17 crawler dozer application.

What is the operating weight of the machine it powers?

The base Komatsu D50A-17 configuration runs about 12,240 kg. Weight varies with blade and undercarriage choice, and the D50P-17/D50PL-17 low-ground-pressure swamp variants run heavier due to their wider track and LGP undercarriage.

What replaced the D50A-17/6D125-1A powertrain?

Komatsu's D50A-18 dozer followed the -17 series, keeping the same 6D125-1 six-cylinder base engine but uprating output to roughly 130 hp (97 kW), before the class moved on to later, electronically-monitored dozer generations.

What 6D125-1A(S/N 10001-UP) owners discuss

What do owners and mechanics say about oil consumption and ring/liner wear on a high-hour 6D125-1A?

On machines well past mid-life hours, oil misting from the crankcase breather and rising oil use per shift get described as a gradual wear-in symptom of tired rings and liners rather than one bad part. Consumption tends to climb once blow-by increases, so the diagnostic people trust is a compression or blow-by check across all six cylinders, not chasing oil brand or filter interval. On an engine this age, that kind of wear is treated as normal end-of-service-life behavior, not a defect.

What fuel-system quirks come up on this era of Komatsu diesel?

This engine runs a mechanical inline injection pump and governor with no electronic controls, and the pump itself is described as rarely the true cause of a power complaint. Mechanics report chasing reported power loss into a full injection pump overhaul, only to find the real fault was a small air leak on the low-pressure transfer/feed line or a tired O-ring on a fuel shutoff or check valve. Fuel that sat in the tank over a season is also called out for growing algae or gel, showing up as dark slime in filters and intermittent fuel starvation. Bleeding and pressure-checking the low side before touching the pump is the common first step.

How does the cooling system behave and what wears out first?

The engine uses wet cylinder liners in direct contact with coolant, and keeping supplemental coolant additive topped up over the service interval is treated as make-or-break maintenance rather than optional. Letting additive deplete lets the liners pit from cavitation, which eventually shows up as coolant leaking into the crankcase (steam off the breather on startup, milky oil). In dusty operating conditions a seized fan clutch is a commonly reported precursor to water pump bearing failure, since a clutch stuck engaged loads the pump shaft with extra axial thrust. A clogged radiator core from dust and chaff is called out as the single most common overheating cause on machines of this class and age.

What powertrain behavior is typical, and what does a steering complaint usually mean?

This dozer uses a mechanical drivetrain with a dry, multi-disc steering clutch and brake arrangement rather than a modern wet system. The classic symptom reported is losing the ability to turn one direction while the machine still tracks straight forward and reverse fine, which points to worn steering clutch discs or a linkage/air-bleed issue at that side, not a transmission or converter fault. Slipping or sluggish response under load more broadly gets traced to low or contaminated clutch/transmission oil before anyone assumes internal damage. Because steering and stopping ability are safety-critical on a tracked machine, have your dealer verify steering clutch and brake condition and adjustment before relying on it.

What undercarriage wear pattern should be expected on this size and class of dozer?

Owners consistently treat undercarriage condition, not the engine, as the biggest value and cost driver on a dozer this size. Sprockets, track links, and rollers wear steadily under sustained pushing and dozing loads, and track sag needs periodic readjustment as pins and bushings wear and stretch the chain. Wear well past the point of easy adjustment is described as expensive enough to change the economics of the whole machine, so undercarriage wear percentage is checked before anything else on a prospective purchase.

Does this engine have sensor or electrical quirks like later Komatsu engines?

No. This is a 1980s-era mechanical diesel with a mechanical governor and inline injection pump, with no electronic engine controls or sensors to fail. What owners call "quirks" on this series are mechanical: a worn governor linkage or weak governor springs causing idle hunting or surging, and valve lash that has to be reset by hand on a schedule since there is no self-adjusting valve gear. Treat any described "sensor problem" on this engine with suspicion, since there is no ECM or sensor network for it to belong to.

What should a buyer check on a used D50A-17/D50P-17/D50PL-17 with this 6D125-1A engine?

Confirm the engine and machine serial numbers line up with a documented history, since Komatsu reused this engine family across several dozer and other equipment variants and mismatched running gear is a known grey-market risk on older machines. Beyond that, the checklist mirrors what owners repeat for machines of this age and class: undercarriage wear percentage on sprockets, links, and rollers; steering clutch and brake operation in both directions under load; hydraulic pump output and blade/tilt cylinder drift; coolant checked for an oil sheen and oil checked for milkiness or a coolant smell; and even compression or blow-by across all cylinders. Parts and documentation support for a machine this old runs thinner than for contemporary equivalents, so budget extra lead time for sourcing used or rebuilt undercarriage and final drive components.

Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.

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